Electrically conductive pavement mixture

ABSTRACT

An electrically conductive paving system that has enhanced conductivity of the conductive graphite/asphalt layer. This is achieved by incorporating into the paving mixture a blend of two naturally occurring crystalline flake graphites, one being coarse (generally larger than 40 mesh/425 microns) and the other being fine (generally smaller than 100-200 mesh/150-75 microns). The ratio of course flake graphite to fine flake graphite may be from 1.5:1 and 1:1.5, but is preferably approximately 1:1.

This application claims the benefit of Provisional Application No.60/249,241 filed Nov. 16, 2000.

BACKGROUND OF THE INVENTION

It is known to use electrically conductive paving mixtures as part of apavement system to prevent the accumulation of frozen precipitation byuse of electrically-generated heat.

In our prior U.S. Pat. No. 5,707,171, an electrically conductive pavingsystem is disclosed that comprises a grid of electrically conductivecables, a layer of electrically conductive paving mixture, an electricalpower supply, and a monitoring system. The paving mixture comprises anaggregate fraction, a bituminous fraction, and a fraction of blendedgraphite particles, the graphite particles including anaturally-occurring portion and a synthetically-produced portion in aratio of 2:1.

The paving mixture of the prior patent was designed to have sufficientstability and strength for use on airport runways. This requiredcompromising the conductivity characteristics of the paving mixture inorder to achieve sufficient stability and strength. If the electricallyconductive paving is to be used in a less demanding environment, such aspedestrian walkways or lightly-traveled roadways, the stability/strengthrequirements of the pavement becomes less constraining.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross sectional view of an electrically conductive pavementsystem according to the present invention.

DETAILED DESCRIPTION

In the present invention, the paving system has been improved to enhanceconductivity of the conductive graphite/asphalt layer. This is achievedby replacing the blend of two graphites, one naturally occurring and theother synthetically produced, with a blend of two naturally occurringcrystalline flake graphites, one being coarse (generally larger than 40mesh/425 microns) and the other being fine (generally smaller than100-200 mesh/150-75 microns). The ratio of course flake graphite to fineflake graphite may be from 1.5:1 to 1:1.5, but is preferablyapproximately 1:1. The increased conductivity of the graphite/asphaltlayer enables the user to optimize the efficiency in the design of theelectrically conductive paving system for increased heat transfer.

In one embodiment of this invention, the relative proportion of thecoarser sized natural crystalline flake graphite (commercially availablefrom the Superior Graphite Co. of Chicago, Ill., USA as grade 3298) andthe finer sized natural crystalline flake graphite (Superior GraphiteCo. grade −190) is 1:1. The mixture was designed in accordance withAsphalt Institute MS-2 “Mix Design Methods for Asphalt Concrete andother Hot Mix Types.” When added to the bituminous fraction, the blendedgraphite particles comprise approximately 10-25 percent, by weight, ofthe electrically conductive paving mixture.

The size characteristics of the two natural crystalline flake graphitesused in the above-described embodiment are as follows: The coarser sizednatural crystalline flake graphite (Superior Graphite grade 3298) has agradation of not less than 95% passing a U.S. Standard 20 mesh (850micron) sieve and a minimum of at least 80% being retained on a U.S.Standard 35 mesh (500 micron) sieve. The finer sized natural crystallineflake graphite (Superior Graphite grade −190) has a gradation of atleast 80% minimum passing a U.S. Standard 100 mesh (150 micron) sieve.

The resistivity measurements, based on previous experience, indicatethat the amount of graphite can be reduced to at least 15 percent of theaggregate and still achieve excellent resistivity with stabilitysuitable for pedestrian applications and normal void content. Theoptimum bitumen content was found to be 7.2 percent of the aggregate,based on this new graphite blend. Graphites and mixes will changedepending on the anticipated use for the system, as well as theaggregate used at each installation site.

The application and operation of a pavement system utilizing theabove-described paving mixture is generally in accordance with thetechnique set forth in our prior U.S. Pat. No. 5,707,171, which isincorporated herein by reference. With reference to FIG. 1, a schematicview of an electrically-conducted pavement system adapted for using thepresent invention is shown. A base pavement 10, generally on the orderof 50 mm thick, is layed over a base layer 12. On top of the basepavement 10 the conductive layer 14 is layed. The conductive layer 14 isalso on the order of 50 mm thick and includes a grid ofelectrically-conductive cables 16, preferably made of copper.

In the illustrated installation, a waterproof membrane 18 of anasphalt-impregnated fabric substantially surrounds the conductive layer.This layer, preferably comprised of a non-woven fabric commonly used inroadway construction, provides additional insulative protection,increased durability, and improved resistance to water seepage.

A layer of sand 20 is placed over the conductive layer and concretepavers 22 are placed on top thereof to complete the pavement system.

Accordingly, an electrically conductive paving mixture and system havebeen provided that meet all the objectives of the invention. While theinvention has been described in terms of certain preferred embodiments,there is no intent to limit the invention to the same. Instead, theinvention is defined by the following claims.

1. An electrically conductive paving mixture comprising: an aggregate fraction; a bituminous fraction; and a fraction of blended natural flake graphite, the blended natural flake graphite comprising a coarse portion of which is greater than 425 microns in size and a fine portion of which is less than 200 150 microns in size, the coarse portion and the fine portion being combined in a ratio of between 1.5:1 and 1:1.5 and comprising 10-25 percent by weight of the paving mixture.
 2. The electrically conductive paving mixture of claim 1 wherein at least 95 percent of the coarse portion is smaller than 850 microns and at least 80 percent of the coarse portion is larger than 500 microns, while at least 80 percent of the fine portion is smaller than 150 microns.
 3. An electrically conductive pavement system comprising: a grid of electrically conductive cables, an electrically conductive paving mixture covering and surrounding the grid; the paving mixture having an aggregate fraction, a bituminous fraction, and a fraction of natural flake graphite comprising a coarse portion of which is greater than 425 microns in size and a fine portion of which is less than 200 150 microns in seizes size, the coarse portion and the fine portion being combined in a ratio of between 1.5:1 and 1:1.5 and comprising 10-25 percent by weight of the paving mixture; and an asphalt-impregnated fabric encapsulating the conductive pavement.
 4. The electrically conductive pavement system of claim 3 wherein the coarse portion and the fine portion of the graphite fraction are combined in a 1:1 ratio.
 5. The electrically conductive pavement system of claim 3 wherein at least 95 percent of the coarse portion is smaller than 850 microns and at least 80 percent of the coarse portion is larger than 500 microns, while at least 80 percent of the fine portion is smaller than 150 microns. 